The present invention relates to a safety valve for extraction wells of hydrocarbons, such as, for example, extraction wells of petroleum and/or natural gas. In particular, the present invention relates to a safety valve to be installed on the well head beneath the conventional safety systems, called “blow-out preventers” or BOPs, or below the production cross. The present invention is to be used in the case of emergency during drilling, production and maintenance operations.
An extraction well of hydrocarbons is similar to a pipe having a substantially circular section or, in other words, a long pipeline. During drilling, the formation fluids are contained in the pores of the subsoil rock, they are subjected to the formation pressure and are retained in the rock by the counter-pressure exerted on the walls of the well hole by the drilling muds.
Should the formation fluids rise from the well towards the surface uncontrollably, there would be an eruption (blow-out) of said fluids in correspondence with the drilling plant, which is usually situated at the surface on the well head.
The systems currently used for the prevention of blow-out prevalently consist in the installation of blow-out preventers or BOPs. BOPs consist of a certain number of devices called “rams”, configured for surrounding the tubular drilling material. Ram devices are capable of exerting, by means of a suitable element made of metallic or elastomeric material, a closing and hydraulic sealing action on the tubular drilling material present in their interior without necessarily shearing the tubular drilling material itself. Some ram devices, called “blind”, close the well if no tubular material is present in the BOPs. Other ram devices which can be activated as an extreme possibility are so-called “shear rams” (shearing) which shear the drill pipe and allow the closing and sealing element to be inserted.
At present, blow-out preventers or BOPs have the following limits:                they are capable of shearing only the body of the drill pipes; they cannot shear the ends, called “tool joints” of said pipes, which have a larger diameter with respect to the pipe bodies;        they require maintenance and substitution of the sealing elements at the end of the drilling phases;        in the case of shear rams, the shearing action is only optimal when the pipe is centered inside the BOPs.        
BOPs also have additional problems in critical situations. If the drill string is compressed upwards by the well pressure, or if it is diverted laterally, for example, the type of shearing of the BOPs may not ensure the shearing of the drill pipes and the passage of the closing element. Furthermore, the passage of the shear rams envisages that the pipe be cut after a complete crushing of the section, which only takes place in the central part of the pipe. Finally, the area of the tool joints subjected to the action of the cutting edges of the shear rams tends to break with reduced crushing and with completely unforeseeable fracture lines. Some metal debris may therefore remain entrapped, blocking the ram devices and consequently preventing the closing of the well.